Showing papers on "Near and far field published in 1998"

Representation of electromagnetic fields over arbitrary surfaces by a finite and nonredundant number of samples

Abstract: It is shown that the electromagnetic (EM) field, radiated or scattered by bounded sources, can be accurately represented over a substantially arbitrary surface by a finite number of samples even when the observation domain is unbounded. The number of required samples is nonredundant and essentially coincident with the number of degrees of freedom of the field. This result relies on the extraction of a proper phase factor from the field expression and on the use of appropriate coordinates to parameterize the domain. It is demonstrated that the number of degrees of freedom is independent of the observation domain and depends only on the source geometry. The case of spheroidal sources and observation domains with rotational symmetry is analyzed in detail and the particular cases of spherical and planar sources are explicitly considered. For these geometries, precise and fast sampling algorithms of central type are presented, which allow an efficient recovery of EM fields from a nonredundant finite number of samples. Such algorithms are stable with respect to random errors affecting the data.

High definition aperture probes for near-field optical microscopy fabricated by focused ion beam milling

Abstract: We have improved the optical characteristics of aluminum-coated fiber probes used in near-field scanning optical microscopy by milling with a focused ion beam. This treatment produces a flat-end face free of aluminum grains, containing a well- defined circularly-symmetric aperture with controllable diameter down to 20 nm. The polarization behavior of the tips is circularly symmetric with a polarization ratio exceeding 1:100. The improved imaging characteristics are demonstrated by measuring single molecule fluorescence. Count rates increase more than o­ne order of magnitude over unmodified probes, and the molecule images map a spatial electric field distribution of the aperture in agreement with calculations. (C) 1998 American Institute of Physics.

Method for near field electromagnetic proximity determination for guidance of a borehole drill

Abstract: A method and apparatus for precise measurement of the distance and direction from a magnetic field sensor to a nearby magnetic field source includes an elongated iron core solenoid driven by a repetitive, nonsinusoidal current source. In the near field the solenoid has two spaced, temporally varying magnetic poles, and measurement of the distance and direction to this source includes analysis of field components which vary in synchronism with the current source.

Methods and systems for introducing electromagnetic radiation into photonic crystals

Abstract: A method for introducing desired electromagnetic radiation into a photonic crystal having a photonic bandgap and at least one defect, wherein the desired electromagnetic radiation has a frequency within the photonic bandgap. The method includes the steps of: delivering source electromagnetic radiation having an electromagnetic frequency outside the photonic bandgap to the defect; and generating the desired electromagnetic radiation at the defect in response to the source electromagnetic radiation.

Computation of the near-field pattern with the coupled-wave method for transverse magnetic polarization

Abstract: In this paper, an accurate method for computing the elecromagnetic field distribution in lamellar gratings is proposed. The method that relies on the rigorous coupled-wave analysis provides accurate numerical results and avoids possible sources of artefacts due to permittivity discontinuities. Its performance is analysed through various lamellar grating cases, including dielectric and metallic materials, the visible and near-infrared (1–10 μm) regions of the spectrum. Special attention is placed on field singularities which are in general present at the grating wedges for transverse magnetic polarization.

Reciprocity of evanescent electromagnetic waves

Abstract: We derive reciprocity relations for the generalized reflection and transmission coefficients of vector wave fields containing evanescent components. This is done by using Lorentz’s reciprocity theorem with sources at finite distance from the scatterer.

Electronic and electromagnetic properties of nanotubes

Abstract: A nanotube is phenomenologically modeled as a chain of atoms wrapped helically on a right circular cylinder. The semiclassical Hamiltonian of an electron is derived, using the Wannier approach for the Schr\"odinger equation, when the nanotube is exposed to both constant (dc) and high-frequency (ac) electromagnetic fields. The Boltzmann kinetic equation is then solved in the framework of momentum-independent relaxation time approximation. An analytical expression for electric current in a nanotube is derived. The interaction of nonlinearity and chirality is analyzed, chiefly as the dependence of a current chiral angle on the amplitude of the ac electric field. The derived expressions for the electronic transport also help in stating anisotropic impedance boundary conditions on the nanotube surface. Surface wave propagation in a carbon nanotube (CN) is examined. The idea of using CN's as nanowaveguides in the infrared frequency range is established. Convective instability is shown to occur under special conditions in a CN exposed to an axial dc electric field.

Information content of Born scattered fields: results in the circular cylindrical case

Abstract: In this analysis some limitations of the linear Born approximation in the diffraction tomography problem from far-zone data are pointed out. The analysis is performed by means of singular-value decomposition of the scattering operator in the scalar two-dimensional case of a circular dielectric cylinder illuminated by a TM-polarized plane wave. It is shown that the validity of the Born approximation entails the important condition that the scattering object not present too-fast spatial variations of the permittivity profile. For the rotationally symmetric cylinder, evidence is presented that the imaginary part of the normalized scattered far field has no information content for real permittivity objects. Moreover, for angularly varying cylinders the information content of the scattered far field for a single view is approximately the same as in the multiview case. Examples of singular-value and singular-function behavior and of profile reconstruction are depicted for the considered geometries.

Miniature electric near-field probes for measuring 3-D fields in planar microwave circuits

Abstract: Three-dimensional (3-D) electric near-field probes applicable to the 0.05-20-GHz band have been developed, which can measure both the magnitude and the phase of the microwave field inside radio-frequency (RF) and microwave circuits. The field probes have very small dimensions and do not need to be connected to the operating device-under-test (DUT), therefore, the circuit properties are nearly not disturbed by the probe, Investigations on different circuits (e.g., antenna, meander lines, filters, and power amplifiers) show that such near-field probes can be applied not only to simple passive circuits, but also to measure the fields inside complex active circuits. These simple, stable, and cheap field probes are very useful for assisting the design of microwave circuits, antenna diagnostics, and testing products in industry.

Scanning evanescent electro-magnetic microscope

Abstract: A novel scanning microscope is described that uses near-field evanescent electromagnetic waves to probe sample properties. The novel microscope is capable of high resolution imaging and quantitative measurements of the electrical properties of the sample. The inventive scanning evanescent wave electromagnetic microscope (SEMM) can map dielectric constant, tangent loss, conductivity, complex electrical impedance, and other electrical parameters of materials. The quantitative map corresponds to the imaged detail. The novel microscope can be used to measure electrical properties of both dielectric and electrically conducting materials.

Non-disturbing electric field sensor using piezoelectric and converse piezoelectric resonances

Abstract: This invention relates to an electric field sensor for near field measurement. The electric field sensor of the present invention uses both piezoelectric and converse piezoelectric resonances. Composed of no metallic parts, the probe of the sensor minimizes field disturbance. The most distinguished feature of this probe is that a signal is transmitted outside neither electrically nor optically, but mechanically.

Calculation of the field enhancement on laser-illuminated scanning probe tips by the boundary element method

Abstract: In the near field of an illuminated scanning probe tip with a dimension much smaller than a light wavelength a tremendous field enhancement is possible. This effect is similar to field enhancement on small metal particles first in- vestigated in SERS (surface-enhanced Raman spectroscopy). In this article we propose to apply the boundary element method (BEM) to calculate the electric potential and field for an arbitrary tip-sample geometry. BEM works especially well for the Laplace problem with piecewise constant mate- rial properties. We developed a simple and fast program and carried out calculations for some tip-sample configurations.

Helicon wave excitation with rotating antenna fields

Abstract: By using phased bifilar antennas, helicon waves have been excited by applying fields which rotate either in space or in time, or both simultaneously. The direction of rotation is made to favour either m=+1 or m=-1 waves, where m is the azimuthal mode number, and m is measured directly. Rotation in time is found to be more effective than rotation in space and makes possible a direct comparison of excitation. An m=-1 structure was seen only in the antenna near field, while m=+1 modes propagate far downstream. Up to 2 kW of RF power, density profiles and antenna loading measurements show that m<0 (left-hand) waves are poorly coupled, and m=+1 (right-hand) waves are necessary for good plasma production. Loading results indicate that antennas also couple to absorption mechanisms unrelated to helicon waves.

Do evanescent waves contribute to the far field

Abstract: Evanescent waves have become of considerable interest in recent years because of developments in near-field optics. Claims have been made that such waves contribute to the radiation fields of sources and to the far fields of scatterers. We show, by considering a spherical scalar wave and a linear electric dipole field, that these claims are misleading and that such contributions are without physical consequences. Our conclusions apply to a much broader class of fields than those considered in this Letter.

Microwave imaging of the brightness temperature distribution of extended areas in the near and far field using two-dimensional aperture synthesis with high spatial resolution

Abstract: Aperture synthesis is an attractive alternative to conventional passive microwave imaging systems for remote sensing applications Whenever a high spatial resolution is desired or the imaging process should work like an optical camera or when microwave remote sensing in the near field has to be distance adaptive, conventional systems cause a lot of problems However, aperture synthesis for practical applications requires a lot of single receivers and correlators This can cause other problems because of the large number of possible imperfections To get an estimation of such effects, the imaging expressions for the aperture synthesis principle are developed for the near- and far-field conditions of a real system To support the theory and to get an estimation of the feasibility of this imaging method, a simple experimental system is described and experimental results of high spatial resolution for the near and far field are shown For the near-field case a special reconstruction algorithm was developed and is described theoretically and validated by the experiments A discussion of the temperature resolution of aperture synthesis concludes the paper

Electrooptic mapping of near-field distributions in integrated microwave circuits

Abstract: A field mapping system based on external electrooptic sampling has been developed in order to determine the vectorial components of the electric near-field distribution within microwave integrated circuits. The capabilities of the setup are demonstrated by two-dimensional measurements of normal and tangential fields in a coplanar microwave distribution network at frequencies up to 15 GHz. Results obtained on a functioning power-distribution network, as well as on two nonfunctioning networks, show the ability of the technique to interrogate internal circuit operation and to isolate faults through investigation of the field distributions.

Imaging profiles of light intensity in the near field: applications to phase-shift photolithography.

Abstract: We describe a method of imaging the intensity profiles of light in near-field lithographic experiments directly by using a sensitive photoresist. This technique was applied to a detailed study of the irradiance distribution in the optical near field with contact-mode photolithography carried out by use of elastomeric phase masks. The experimental patterns in the photoresist determined by scanning electron microscopy and atomic force microscopy were compared with the corresponding theoretical profiles of intensity calculated by use of a simple scalar analysis; the two correlate well. This comparison makes it possible to improve the theoretical models of irradiance distribution in the near field. Analysis of the images highlights issues in the experimental design, provides a means for the optimization of this technique, and extends its application to the successful fabrication of test structures with linewidths of ∼50 nm.

Aperture placement effects in oxide-defined vertical-cavity surface-emitting lasers

Abstract: A study of the effects of aperture placement on the properties of vertical-cavity surface-emitting lasers (VCSELs) is presented. When thin apertures are placed at the peak of the electric field standing wave optical losses are very high for small apertures. The threshold current increases with decreasing aperture size and two-dimensional diffraction like patterns are evident in the far field. For apertures placed at a node, optical losses appear to be negligible, and loss of optical confinement is apparent for apertures below 2 /spl pi/m.

Calculations of the direct electromagnetic enhancement in surface enhanced Raman scattering on random self-affine fractal metal surfaces

Abstract: We study the classical electromagnetic enhancement at the excitation wavelength related to surface enhanced Raman scattering (SERS) experimental configurations in the vicinity of random rough metal surfaces possessing self-affine scaling behavior. The scattered electromagnetic intensity is obtained by means of numerical calculations based on the rigorous integral equations formulation of the electromagnetic wave scattering, free from the limitations of electrostatic and/or dipolar approximations. From the enhancement of the scattered field intensity in the immediate vicinity of the surface, originated in the excitation of transversal-magnetic surface plasmon polaritons, the SERS electromagnetic mechanism on substrates of Ag, Au, and Cu is explored as a function of the surface fractal dimension, rms height, and excitation wavelength. It is found that fractality favors the occurrence of large electromagnetic enhancements, which in turn appear to be maximum at an optimum wavelength as a result of the comprom...

Calibration of electric coaxial near-field probes and applications

Abstract: A new calibration technique for application to near-field probes has been developed For this, a simple electric coaxial near-field probe for application in the 005-20-GHz band has been developed, theoretically analyzed, and calibrated using a known field By using the finite-difference time-domain (FDTD) method, this field probe is theoretically analyzed to determine its most sensitive probe segment Taking the amplitude of the normal electric field at this segment as a known field, the probe is calibrated by defining a performance factor (PF), which is the ratio of the known field amplitude to the probe signal amplitude Comparing the calculated results with measured results, the agreement is good Additionally, the measurements are experimentally characterized with respect to the influence of the distance between the probe and the device-under-test (DUT), the influence of the input signal on the probe, and the spatial resolution Two measurement examples are demonstrated, which investigate the mode and field distribution within passive coplanar microwave components

Imaging Microwave Electric Fields Using a Near-Field Scanning Microwave Microscope

Abstract: By scanning a fine open-ended coaxial probe above an operating microwave device, we image local electric fields generated by the device at microwave frequencies. The probe is sensitive to the electric flux normal to the face of its center conductor, allowing different components of the field to be imaged by orienting the probe appropriately. Using a simple model of the microscope, we are able to interpret the system's output and determine the magnitude of the electric field at the probe tip. We show images of electric field components above a copper microstrip transmission line driven at 8 GHz, with a spatial resolution of approximately 200 $\mu$m.

Combined millimeter-wave near-field microscope and capacitance distance control for the quantitative mapping of sheet resistance of conducting layers

Abstract: We present a dual-frequency electromagnetic scanning probe and apply it for quantitative mapping of the sheet resistance of conducting films. The high-frequency (82 GHz) mode is used for image acquisition, while the low-frequency (5 MHz) mode is used for distance control. We measure magnitude and phase of the near-field microwave reflectivity from conducting films of varying thickness and develop a model which accounts fairly well for our results. This brings us to a quantitative understanding of the contrast in the microwave near-field imaging using an aperture probe, and allows us to achieve quantitative contactless characterization of conducting layers with sheet resistance even below 2 Ω.

Near-field propagation of terahertz pulses from a large-aperture antenna

Abstract: The near-field propagation behavior of terahertz (THz) pulses generated by a planar large-aperture photoconducting THz transmitter has been characterized. A simulation model based on Huygens–Fresnel diffraction theory has been developed that permits accurate prediction of the spatiotemporal profiles of the THz beam everywhere and gives excellent agreement with experimental measurements. Two key conclusions emerge from this research, namely, the realization that for practical laboratory setups one is always working in the near-field regime and that the proper temporal shape of the THz field at the antenna is one that rises rapidly but decays slowly.

NF-FF transformation with cylindrical scanning: an effective technique for elongated antennas

Abstract: A sampling representation, particularly suitable for dealing with elongated antennas, is proposed for fast and accurate interpolation of the electromagnetic field radiated on a cylinder in the near field region and properly exploited to develop a new near field-far field transformation technique with cylindrical scanning. The technique relies on recent theoretical results concerning the nonredundant representation of radiated or scattered fields, and allows one to evaluate the antenna far field from minimal near field data. In particular, the proposed technique allows one to reduce significantly the amount of data needed, without losing the numerical efficiency of previous approaches. As a consequence, measurement time and memory storage requirements are reduced remarkably. Numerical tests confirm the effectiveness and stability of the proposed approach.

The specific absorption rate in a spherical head model from a dipole with metallic walls nearby

Abstract: The head of the user of a mobile telephone is modeled by a lossy dielectric sphere excited by the near field of a dipole antenna and the effects of a metallic wall placed close to this system are analyzed. Three wall orientations are used and the changes in specific absorption rate (SAR) with wall or antenna distance from the sphere are investigated. When the wall is closer than 100 mm to the dipole-sphere system, it can have a strong influence on the input impedance of the antenna and can cause significant increases in the SAR in the head. When a constant radiated power of the dipole antenna is assumed, the maximum value of SAR averaged over 10 g of tissue within the sphere is increased by 60% when the wall is as close as 30 mm from the surface of the sphere. For a dipole excited by a constant current source, the increase is less than 10% and occurs when the wall is 120 mm from the surface of the sphere. Although the distances and configurations analyzed may not represent the way the majority of people use mobile telephones, it is important to examine unusual situations to determine the extent of the SAR enhancement problem.

Treatment of retardation effects in calculating the radiated electromagnetic fields from the lightning discharge

Abstract: The analytical treatment of retardation effects in calculating lightning electromagnetic fields far from the source has often involved the use of a so-called F factor. The literature concerning the ...